1. Field of the Invention
This invention relates to a method of determining hardness of a wellbore fluid filtrate, and to a composition for determining hardness of a wellbore fluid filtrate.
2. Background Art
Wellbore fluids, commonly referred to as “Mud” provide a number of functions when drilling a wellbore. Wellbore fluids may comprise a slurry of various components in which a solid material is dispersed in a liquid. Filtration of the wellbore fluid thus produces a wellbore fluid filtrate. To insure performance and the integrity of the system, the properties and compositions of wellbore fluids are monitored. One important property of a wellbore fluid is the hardness of the wellbore fluid filtrate, which is a measure of the concentration of divalent cations, typically calcium ions (Ca2+), magnesium ions (Mg2+), zinc ions (Zn2+), and others known to one of minimal skill in the art.
Numerous methods exist to determine the hardness of a wellbore fluid filtrate. One common method includes a titration of an aliquot of the wellbore fluid filtrate using a buffer solution and a colorimetric indicator. Colorimetric indicators are supplied as solutions of the particular indicator dissolved in a buffer, with or without a solvent. Colorimetric indicator solutions useful in determining the hardness of a wellbore fluid filtrate, however, are plagued with short shelf lives and less than ideal toxicity profiles, mainly due to the buffer solutions and/or solvents utilized in the solutions. These issues are exacerbated in oilfield applications, wherein the indicators may be subjected to extreme temperatures, and/or long shelf times due to the remote nature of the enterprise.
Known indicators for determining hardness in a wellbore fluid filtrate include Versenate indicator solution, which comprises calmagite [(1-Naphthalenesulfonic acid, 3-hydroxy-4-[(2-hydroxy-5-methylphenyl)azo] (CAS 3147-14-6)], in a solution of ethylene diamine tetraacetic acid (EDTA), at a pH of greater than 9 provided by a NaOH/NaOAc buffer solution or a NH4OH/NH4Cl buffer solution. This solution may further include ethylene glycol and/or propylene glycol as both a solvent and an antifreeze to preserve the solution.
Other indicators comprise Eriochrome Black T indicator (sodium 3-hydroxy-4-[(1-hydroxy-2-naphthyl)azo]-7-nitronaphthalene-1-sulphonate), (CAS 1787-61-7). However, aqueous solutions of Eriochrome Black T are unstable. Accordingly, Eriochrome Black T is provided in a solution commonly referred to in the art as Manver indicator solution. Manver indicator solution typically comprises about 0.1 wt % Eriochrome Black T dissolved in about 90% triethanolamine and about 10% ethanol. The triethanolamine is present to stabilize Eriochrome Black T. Triethanolamine, however may present various health and safety issues.
Accordingly, there is a need for an indicator useful in determining the hardness of a wellbore fluid filtrate. The indicator preferably has a long shelf life, a suitable toxicity profile, and provides both precision and accuracy in determining the hardness of a wellbore fluid filtrate.